In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized. This can be accomplished by an early and therefore higher opening of the exhaust passage into the cylinder. The adjustment of the exhaust port, to obtain maximum power capacity of the engine at high speeds involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with a lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston which means that the exhaust port must be at a lower position than it is at high engine speeds.
For this purpose it is known to provide, in the exhaust port, a valve which is movable between a full flow position and a flow restricting position. When in the flow restricting position, the end of the valve is substantially flush with the peripheral surface of the cylinder bore. In this flow restricting position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The valve is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine.
Over the course of the engine's life, a process known as “coking” occurs, in which byproducts of fuel combustion in the cylinders are deposited and accumulate on various components in the exhaust path of the engine, including the movable valve. The accumulation of coking deposits can impede the movement of the valve between the full flow position and the flow restricting position. As a result, the actual position of the valve may differ from the desired position, resulting in reduced engine performance. In addition, if the valve is not in the desired position due to coking or other impediments, its actual position is not easily determinable and it is difficult to adjust either the valve position or other aspects of engine performance such as the fuel-air ratio to be appropriate for the actual valve position. In addition, coking in one or more cylinders of a multi-cylinder engine may result in the valves of two or more different cylinders being in different positions. As a result, some of the cylinders will have different performance characteristics, which can adversely affect engine performance, fuel economy and rider comfort, as well as causing premature wear to engine parts.
Therefore, there is a need for a movable valve assembly suitable for use with two or more exhaust ports of a two-stroke internal combustion engine with a reduced likelihood of having its movement impeded due to coking.
There is also a need for a movable valve assembly suitable for use with two or more exhaust ports of a two-stroke internal combustion engine, wherein a difference in position between an actual position of the valves and a desired position of the valves can be determined.
There is also a need for a movable valve assembly suitable for use with two or more exhaust ports of a two-stroke internal combustion engine, wherein discrepancies in valve position between the exhaust ports are reduced.